The foregoing disclosure sets forth a pitch which enables carbon fibers to be made by drawing. The carbon fibers made from the pitch have physical characteristics which are in part described by the foregoing disclosure. Separate from that disclosure, it has been additionally discovered that a carbon fiber made with the pitch set forth in that disclosure can be treated during processing to thereby obtain an elastic fiber.
Elastic fibers find great use and application in various and sundry yarn manufacturing procedures and also in the making of cloth or fabrics from such yarns. Yarn manufacturing equipment involves rough handling of the fibers used to make the yarn, and subsequent processing by other equipment is also rough on the fibers making the yarn and cloth made therefrom. For these reasons, it is very important to have a fiber which is not brittle and which has a minimum measure of elongation. Most yarn manufacturing equipment requires a minimum of about two percent elongation of the individual fibers to enable the machinery to operate properly. An alternate use of the fiber of the present invention is shown in U.S. Pat. No. 4,647,360, assigned to present assignee.
It has been discovered that controllable elongation in carbon fibers is very important to obtaining a suitable fiber, that is, a fiber which can be used by typical yarn and cloth manufacturing equipment. The present fiber qualifies on these important criteria and is thus described as a fiber which is suitable for spinning and weaving. Moreover, the fiber of the present disclosure and the method of manufacture for making the fiber describes a fiber which has a controllably high percent elongation The physical characteristics of the fiber of this disclosure can be modified over a range by post-manufacture treatment to increase the percent elongation. In such an instance; controllable elongation can be obtained. An important feature of the present disclosure is the provision of a fiber which has a significant elastic range with substantially no plastic range. This defines a fiber which can be relied on to return to its original length after stressing. On return to its original length, it still has approximately the same load-bearing capacity, this capacity being unchanged by virtue of the fact that there is no plastic elongation in the fiber.
With the foregoing in mind, the present disclosure is thus summarized as a method of manufacture of a special fiber and the fiber having specific physical characteristics. The method of manufacture utilizes a pitch of a specific description for melt spinning to form a fiber and subsequent treatment after drawing. The post drawing treatment includes oxidizing, preferably with a chlorine atmosphere for a specified interval. Other oxidants can be used including oxygen or air mixed with NO.sub.2 serving as a catalyst. The weight of the fiber is increased by chlorine oxidation preferably to about 20 percent, and within the range of about 15 to about 40 percent. Extent of oxidation is an important process parameter; percent weight gain is a rough estimate of oxidation. The percent increase is different with different oxidants. Further, oxidation can be evaluated by factors other than percent weight change. After oxidation with a selected oxidant and the subsequent increase in weight, the fiber is then subjected to a heated atmosphere, steadily increased to avoid fiber destruction in a processing step known as dehydrohalogenation. The processing step is labeled differently should a nonhalogenic oxidant is used. This then yields a fiber having a relatively large percent elongation, controllably typically in the range of 2 up to a larger percent. The elongation can be increased thereafter by repetitively stressing the fiber through several cycles (typically 3 l to 10) wherein the stress applied is perhaps 25-75 percent of the tensile strength of the fiber. After stressing, the fiber will evidence an increased percent elongation, typically above 5 percent and perhaps as high as 7 percent. At this juncture, the fiber may still be described as responding as an elastic member having substantially no performance in the plastic range.